In the field of signal transmission, there are generally two methods of transmission: one being that transfer clock signals and clock signals of a sampling interval are transmitted as well as data to be transmitted; and the other being that data are modulated and transmitted over a single data line. Especially, the latter is further classified into two methods: (A) a synchronizing pattern as shown in FIG. 1 is added to every data block prior to modulation and (B) a synchronizing interval which is a violation of the modulation rule as indicated in FIG. 2 is provided for every data block. The modulation rule used herein is the Bi-phase method, for example. The method (A) requires a pattern matching circuit on the demodulator side for detecting the synchronizing pattern and incurs the risk of causing an error in detection of a sampling interval in the event that a data block happens to bear the same pattern as the synchronizing patterns. In addition, the method (B) has problems of expansion of a frequency band necessary for transmission and decline of the capacity of data transmission, because of the violation of the modulation rule.